1. Technical Field
The present invention relates to a fixing apparatus or the like to fix a toner image onto a recording material in an image forming apparatus utilizing e.g. an electrophotographic method.
2. Related Art
Generally, in an image forming apparatus using powder toner, at a process to fix a toner image, a method of electrostatically transferring a toner image onto a recording medium, then placing the recording medium between a heating member and a pressure member, and heat-melting the toner image thereby press-fixing the toner image to the recording medium, is widely employed. For the heating of the heating member, an arrangement where the heating member has a conductive layer such that the conductive layer generates heat by electromagnetic induction heating has been proposed. The electromagnetic induction heating is providing an exciting coil to generate a varying magnetic field near the conductive layer (heating member) and causing the conductive layer to generate heat by an eddy current generated in the conductive layer. According to the electromagnetic induction heating, as the heating member is directly heated and the range of high temperature by heating is extremely limited, the heating member can be heated to a predetermined temperature in a short time. Accordingly, in comparison with heating using a halogen lamp or the like as a heating source, warm-up time of the fixing apparatus can be reduced, and electric consumption can be reduced. Further, as it is not necessary to previously heat the heating member when the apparatus is not used, the electric consumption can be further reduced.
On the other hand, as the heating member (fixing member), as well as a heating roller, an endless fixing belt is generally used. The endless fixing belt is a belt put around plural support rollers, or is a belt with an inside pressure member and is circulate-driven without a roller. The fixing belt has a thin heat-resisting resin layer or the like as a base layer. As the thermal capacity of the fixing belt is smaller than that of the heating roller, the warm-up time is shorter in comparison with that of the apparatus using the heating roller. Further, in the non-expanded type fixing belt, the area to be contact with another member can be reduced, thereby heat transfer to the other member can be reduced. Accordingly, further efficient warming up can be performed.
In a fixing apparatus where an endless belt as a heating member is heated by electromagnetic induction, when the endless belt is put around plural rollers, the exciting coil is provided to face the inner surface or outer surface of the belt. On the other hand, when the endless belt is circulate-driven without a roller, the exciting coil is provided in a position close and facing the outer peripheral surface of the endless belt. Then, a varying magnetic field is generated in a direction through the endless belt, and an eddy current is induced around the magnetic field.
Generally, a high frequency current supplied to the exciting coil is generated by switching a direct current at a high frequency, and constant current control or constant energy control is performed. Further, upon electric power supply to the exciting coil, the temperature of the fixing member as a heated body is detected with a temperature sensor and the amount of supplied power is controlled and/or power supply ON/OFF control is performed so as to maintain a predetermined temperature.
In recent image forming apparatuses, further reduction of warm-up time is needed. Accordingly, when a printout request has been inputted from a user, it is necessary to immediately heat the fixing apparatus (heating member) to a fixing temperature.
On the other hand, in a fixing apparatus employing e.g. the electromagnetic induction heating, the warm-up time can be reduced as described above, however, as the temperature of the heating member rises in a short time, it frequently overshoots, i.e., it exceeds the upper limit of a desired temperature range. When the overshoot occurs, the heating member or the like is damaged by the overheating, and the life of the heating member may be reduced or the member may be broken.
To address the above problems, it may be arranged such that the level of electric power supplied to the exciting coil during the warm-up is gradually lowered and the inclination of a heat-up curve of the heating member is gradually reduced, thereby the occurrence of overshoot is suppressed.
However, in this method, the warm-up time of the heating member becomes long, and the advantage of the electromagnetic induction heating cannot be utilized. Further, since the power supplied to the exciting coil is set at multiple levels, the power supply ON/OFF control is frequently performed. In this case, a secondary fault such as flicker easily occurs.
Note that these problems are not limited to the fixing apparatus employing the above-described electromagnetic induction heating but similarly occur in a fixing apparatus having a mechanism to quickly heat the heating member.